1. Field of the Invention
This invention relates to a method for determining 3-dimensional structure in biplane angiography from two biplane images obtained at arbitrary orientations with respect to an object.
2. Discussion of Background
The development of digital imaging techniques in the last twenty years has greatly expanded the field of radiology. Digital subtraction angiography (DSA) makes use of the digital format of the vascular images by the subtraction of a mask frame from an image containing contrast-filled vessels. The result is an image in which intervening structures and background have been removed. At present, DSA images are widely used in the diagnosis and treatment planning of most diseases of vessels, including atherosclerosis, aneurysms, arteriovenous malformations, etc.
The digital format of DSA also lends itself well to quantitative measurements of the vascular system. Many researchers have developed methods using single DSA images to quantify physical parameters such as vessel size, the amount of narrowing (or stenosis) of a vessel, or the rate of blood flow in a given vessel or supplying a given tissue. The application of all such quantitative methods is complicated by the fact that a single projection image of the vasculature provides little information concerning the true 3-dimensional vascular structure. Thus, the magnification of a vessel, which is a function of its relative 3-dimensional position between the x-ray source and the imaging plane, is difficult to derive from a single image. In calculations of vessel size and blood flow rate, the magnification of the vessel enters as the first and third power, respectively. (LE Fencil, et al, Accurate Analysis of Blood Flow and Stenotic Lesions by Using Stereoscopic DSA System, Medical Physics, 1987, 14, p. 460, presented at AAPM, 1987). In addition, the 3-dimensional orientation of the vessel with respect to the imaging plane is difficult or impossible to infer from a single image. Knowledge of the orientation of vessels is important for quantitative blood flow measurement, and is also important for the diagnosis of vessel malformations and for surgical planning.
In short, an accurate 3-dimensional (3-D) representation of the vascular structure would be very useful in many areas of medicine.
Several methods have been developed which derive 3-D information from two digital images. Stereoscopic digital angiography has been used in the calculation of 3-D position and orientation information of vessels (LE Fencil et al., Investigative Radiology, December 1987; and KR Hoffman et al., SPIE Medical Imaging, Vol. 767, p. 449, 1987). However, stereoscopic determination of 3-D vessel position becomes less accurate if the orientation of the vessel is close to the direction of the stereoscopic shift. Thus, the reliability of this method in determining 3-D vascular structure depends on the orientations of the vessels themselves.
Szirtes in U.S. Pat. No. 4,630,203 describes a technique for the 3-D localization of linear contours appearing in two stereoscopic images. However, this method also suffers from the limitation that the contour must not lie in the direction of the stereoscopic shift. In addition, a separate calibration step is required in this method to determine the 3-D locations of the x-ray sources relative to the imaging plane.
Several workers have developed methods to derive 3-D structure from two radiographic images that are obtained in exactly orthogonal directions (A. Dwata et al., World Congress in Medical Physics and Biomedical Engineering, 1985; and JHC Reiber et al., Digital Imaging in Cardiovascular Radioloqy, Georg Thiem Verlag, 1983). The 3-D information obtained with these techniques in binary: i.e., no gray levels remain in the reconstructed image of the 3-D object. Secondly, the images must be obtained in exactly orthogonal directions, which may be difficult to achieve in conventional biplane radiography systems. Also, determination of the positions of vessel segments which run in a direction perpendicular to one of the imaging planes is difficult or impossible with these methods.
To eliminate these problems, a method has been developed that allows calculation of 3-D vascular structure from two images obtained at arbitrary orientations (see JM Rubin et al., Investigative Radiology, Vol. 13, p. 362, 1978; and SA MacKay et al., Computers and Biomedical Research, Vol. 15, p. 455, (1982)). This method is important, but requires a separate, somewhat cumbersome calibration step which is executed either before or after imaging the patient. Specifically, a calibration object of known dimensions is imaged in the same biplane configuration as is used to image the patient. Data are then collected from the images of the calibration object, and parameters are calculated to provide the 3-D positions of selected points in the vascular images.
A different technique has been described in the field of computer vision to determine 3-D positions of object points from two arbitrary views without a calibration step. This method was described in two independent theoretical papers. (HC Longuet-Higgins, Nature, Vol. 293, p. 133, 1981; and RY Tsai, TS Huang, Technical Report, Coordinated Science Laboratory, University of Illinois, Nov. 12, 1981). It does not appear, however, that this approach has ever been successfully applied in the field of radiology.